The use of reflected light received by optical elements, such as optical fibers, employing the principle of total internal reflection to measure the displacement of a target is well known. Known probes generally comprise a bundle of optical fibers, some of which transmit light to a target and some of which conduct light back from the probe face to a light sensitive transducer. A portion of the light that strikes the target is reflected back to the probe. The reflected light is received by and communicated along the optical fibers in the probe to a transducer which then generates an electrical signal proportional to the light received. As the target distance decreases, the intensity of reflected light decreases. Thus changes in the signal output of the light sensor can serve as a measure of changes in target distance. Typical examples of such sensors are disclosed in U.S. Pat. Nos. 3,327,584 to Kissinger, 3,940,608 to Kissinger et al., 4,247,764 to Kissinger, 4,694,160 to Hoogenboom et al., and 4,701,611 to Kissinger. Measurement of displacement based on changes in light intensity significantly limits the usefulness and sensitivity of such known devices. First, different targets have different types of surfaces with different reflectivities. Second, variation in the light source results in variation in the output signal from the transducer. This contributes to error in the measurement reading. A third problem with these designs is that cross talk between light emitting and light receiving fibers in mixed bundles considerably reduces the resolving power of apparatus.
More recently U.S. Pat. Nos. 4,701,610 to Hoogenboom, and 5,017,772 to Hafle disclosed geometries that reduce some of the dependence of the signal on surface reflectance by calculating the ratio of two signals produced by two or more receiving fibers spatially separated from each other. This technique fails to eliminate errors caused by any non-uniformity or patterning on the target surface. Additional rings of fibers with significant spatial separation provided to increase sensitivity undesirably increase the size of the sensor.
U.S. Pat. No. 5,073,027 to Krohn et al. discloses a device for determining position. This apparatus is subject to errors caused by variation in surface reflectivity.
U.S. Pat. No. 4,946,275 to Bartholomew discloses a method that attempts to obtain distance information by using a diffraction grating with white light to project a rainbow on the target surface. The color of the return light as seen by the aperture of an optical fiber will correlate with distance. Precise determination of color wavelength in addition to the influence of the color properties of the target surface will prevent this method from providing an accurate measure of distance.
Another method of non-contact displacement measurement producing very high accuracy, which is also nearly immune to variations in surface reflectivity and surface patterning, is disclosed in U.S. Pat. No. 6,088,110 to Rudd et al., among others. The distance to the surface may be calculated using well-known trigonometric relationships by focusing a laser light to a very small spot size at the target surface and then imaging that spot onto a position-sensitive transducer. Accuracies of 1 part in 2,500 are obtained with ordinary commercial techniques. This means that for short-range devices measurement accuracies of less than plus or minus one micrometer are readily achieved. Devices utilizing this principle, however, are relatively large. These devices cannot be brought to close target distance or into restricted spaces. The large size of the sensing window makes them difficult to apply to in-process measurement. The associated weight of the enclosure containing the optical elements makes them difficult to apply to precision metrology instrumentation. The complex assemblies associated with these devices further restrict them from use in hostile environments such as areas of relatively high or low temperature, or spaces filled with explosive gases.
Thus, there exists a need for a non-contact probe that can determine the displacement of a target accurately over small changes in distance, and which will provide a measurement that is substantially independent of target reflectivity and source illumination. There is an additional need for a sensing probe having the above characteristics that is compact and lightweight. There is also a need to have a distance-measuring probe that can be constructed of materials able to withstand high temperature ranges or an explosive atmosphere environment.